This invention relates to a belt power transmission device capable of automatically balancing belt tension by pressing a span of a power transmission belt, such as a V-ribbed belt, entrained about pulleys with an auto tensioner, and more particularly relates to a technique for suppressing a slip, noise and other adverse effects of the belt that are caused by a rocking motion of the auto tensioner associated with a variation in rotational speed of a drive pulley.
As belt power transmission devices for driving auxiliary machinery which are used to drive auxiliary machinery such as an alternator or a pump for power steering with an engine output, conventionally well known are devices formed by entraining a V-ribbed belt (multiple-ribbed belt) about a crank pulley on a crank shaft of an engine and auxiliary pulleys on input shafts of auxiliary machines.
Furthermore, in recent years, such belt power transmission devices often employ a serpentine drive layout for driving a belt in a manner to entrain the belt about respective pulleys not only with its inner face but also with its outer face, together with an auto tensioner (automatic belt tension balancer), integrated into the belt power transmission device, for automatically balancing belt tension by normally pressing, for example, a slack-side span of a belt. The serpentine drive provides improved maintainability, elongated life and enhanced reliability of the belt.
However, such a serpentine drive has a problem that if a variation in rotational speed of the drive pulley is large (for example, if the peak value of the rate of variation in rotational speed thereof is within the range of 10 to 30%) under particular service conditions of the layout of the auto tensioner, the setting of damping torque and the type of belt for use, then a rocking motion of the auto tensioner (more specifically, a rocking motion of the tension pulley and the arm thereof) and a tension variation of the belt become large.
To cope with the above problem, there has been proposed a technique for restricting elongation of a belt to reduce the tension variation thereof under conditions that variations in rotational speed of the drive pulley are large by setting the belt properties to increase the modulus of longitudinal elasticity (longitudinal modulus) of the belt, as disclosed in Japanese Unexamined Patent Application Gazette No. 5-164206.
While the above proposed technique characterizes the belt properties in relation with the properties of the auto tensioner, it does not clearly disclose the tension variation of the belt and rocking motions of the auto tensioner responsive to elongation of the belt due to the tension variation thereof. Therefore, when the auto tensioner has an extremely small rate of absorption of belt elongation, for example, 0.3 to 1.0 mm/deg, if a rocking motion of the auto tensioner becomes very large due to the layout of the auto tensioner relative to the belt, properties of the engine and the auxiliary machinery, the belt properties, the damping property of the auto tensioner or the like, simply specifying the modulus of longitudinal elasticity of the belt as done in the above proposed technique could not avoid the occurrence of a slip or a frictional sound of the belt and the occurrence of abrasion and a frictional sound of a resin-made friction member of the auto tensioner.
Note that the rate at which the auto tensioner absorbs belt elongation represents the amount of absorption of a length of the belt per unit angle of the arm of the auto tensioner, and is also called the take up rate.
An object of the present invention is to suppress rocking motions of the auto tensioner to reduce the occurrence of a slip and a frictional sound of the belt and the occurrence of abrasion and a frictional sound of the friction member of the auto tensioner by further suitably associating the belt properties with the auto tensioner properties when the auto tensioner has a small rate of absorption of belt elongation.
To attain the above object, the present invention is directed to a belt power transmission device in which a power transmission belt is entrained about a plurality of pulleys including a drive pulley variable in rotational speed and a driven pulley and an auto tensioner is provided for automatically balancing tension of the power transmission belt by normally pressing a span of the belt, and the power transmission belt is characterized in that when the rate at which the auto tensioner absorbs elongation of the power transmission belt is 0.3 to 1.0 mm/deg, the damping coefficient of the auto tensioner is set in accordance with the modulus of longitudinal elasticity of the power transmission belt.
With the above structure, when the rate at which the auto tensioner absorbs elongation of the power transmission belt is as extremely small as 0.3 to 1.0 mm/deg, the damping coefficient of the auto tensioner is set in accordance with the modulus of longitudinal elasticity of the power transmission belt. Therefore, even when the auto tensioner has such an extremely small rate of absorption of elongation of the power transmission belt, a rocking motion of the auto tensioner can be minimized thereby reducing the occurrence of a slip and a frictional sound of the belt and the occurrence of abrasion and a frictional sound of a friction member of the auto tensioner.
The drive pulley may be a crank pulley carried on an output shaft of an engine, the driven pulley may be an auxiliary pulley carried on an input shaft of an auxiliary machine, and the power transmission belt may be a V-ribbed belt entrained about at least the crank pulley and the auxiliary pulley in a serpentine drive layout.
With this arrangement, particularly in the belt power transmission device for driving the auxiliary machinery with the engine, a rocking motion of the auto tensioner can be suppressed thereby reducing the occurrence of a slip and a frictional sound of the belt and the occurrence of abrasion and a frictional sound of a friction member of the auto tensioner.
Furthermore, in the above case, the modulus of longitudinal elasticity of the V-ribbed belt and the damping coefficient of the auto tensioner set in accordance with the modulus may be specified. For example, it is preferable that the modulus of longitudinal elasticity of the V-ribbed belt is at least 9800N/100% rib and less than 29400 N/100% rib and the damping coefficient of the auto tensioner is at least 20% and less than 100%, or that the modulus of longitudinal elasticity of the V-ribbed belt is at least 29400 N/100% rib and less than 73500 N/100% rib and the damping coefficient of the auto tensioner is at least 10% and less than 100%, or that the modulus of longitudinal elasticity of the V-ribbed belt is at least 4900 N/100% rib and less than 9800 N/100% rib and the damping coefficient of the auto tensioner is at least 30% and less than 100%, or that the modulus of longitudinal elasticity of the V-ribbed belt is not more than 4900 N/100% rib and the damping coefficient of the auto tensioner is at least 40% and less than 100%.